Preheat oven to 350°F (175°C).

Melt 3 tablespoons butter in a large skillet over medium heat. Add 3 large yellow onions, 1 teaspoon salt, and 1/2 teaspoon black pepper, and cook, stirring occasionally, for 20 to 25 minutes until the onions are deep golden brown and caramelized.

Reserve three-quarters of the caramelized onions in a bowl and leave the remaining quarter in the skillet.

In a large mixing bowl, combine 2 pounds ground beef, 1 cup panko breadcrumbs, 1/2 cup beef broth, 2 eggs, 1/4 cup minced fresh parsley, 2 minced garlic cloves, 1 teaspoon dried thyme, 1/2 teaspoon dried rosemary, 1 tablespoon Worcestershire sauce, and 3/4 cup shredded Gruyere cheese. Gently mix until just combined.

Fold the reserved caramelized onions into the meat mixture.

Press the meat mixture into a 9x5-inch loaf pan lined with the remaining onions on the bottom.

Sprinkle the remaining 1/4 cup shredded Gruyere cheese on top of the loaf and pour 1/4 cup beef broth into the pan around the meatloaf.

Place the loaf pan on a baking sheet and bake for 45 to 50 minutes, or until the internal temperature reaches 160°F (71°C) and the cheese is golden and bubbly.

Remove the meatloaf from the oven and let it rest for 5 minutes before slicing and serving.